1. Field of the Invention
The present invention relates to improvement of the gel time of unsaturated polyester thermosetting resins containing polymerizable vinyl monomers and a vinylphosphonate flame retardant which normally increases the gel time of such resins.
2. Description of the Prior Art
The use of polymerizable vinyl monomers, such as styrene, and certain vinylphosphonate flame retardants, such as the bis (hydrocarbyl)vinylphosphonates, particularly bis(beta chloroethyl)vinylphosphonate, in unsaturated polyesters is known. However, it has been found that the presence of such a vinylphosphonate flame retardant increases the gel time of the resin formulation in which it is incorporated. Increase of the gel time decreases the production rate of these resin formulations and is a commercial disadvantage.
The prior art is contradictory with respect to its teaching as to the effect maleic anhydride has on polyester systems.
U.S. Pat. No. 3,232,893 shows the use of maleic anhydride as a heat providing agent for vapor expansion and a gel time quickener in foamable polyester formulations which may contain conventional flame retardants which do not retard the gel time of such resins
U.S. Pat. No. 3,832,268 shows a reinforced plastic structure which is formed from a thermoplastic substrate having bonded thereto a cured polyester and a monomer mixture of a copolymerizable unsaturated carboxylic acid or anhydride, e.g., maleic anhydride, and a different copolymerizable monomer. e.g., styrene. The polyester does not contain any flame retardant additive.
U.S. Pat. No. 2,479,486 shows a styrene and maleic anhydride-containing polyester composition wherein some of the ester portion of the polyester is replaced with an anhydride of tetrahydrophthalic acid derived from maleic anhydride and butadiene. These compositions are said to polymerize quickly at low temperatures. The composition shown in this patent also fails to contain any flame retardant additive.
British Patent No. 974,214 relates to polyesters containing a vinyl monomer, e.g., styrene, a radical forming agent, an accelerator, and either itaconic, citraconic or maleic anhydride. The maleic anhydride is preferred and is said to increase, rather than decrease, the gel time of the resulting compositions. This patent fails to suggest the presence of a flame retardant additive in the polyester composition.